Ultrasonic detection device and ultrasonic diagnostic device

ABSTRACT

Provided is an ultrasonic detection device including: a capacitive electromechanical transducer including a cell that includes a first electrode and a second electrode disposed so as to oppose with a space; a voltage source for developing a potential difference between the first electrode and the second electrode; and an electric circuit for converting a current, which is caused by a change in electrostatic capacitance between the first electrode and the second electrode due to vibration of the second electrode, into a voltage, in which the capacitive electromechanical transducer provides an output current with a high-pass characteristic having a first cutoff frequency with respect to a frequency, the electric circuit provides an output with a low-pass characteristic having a second cutoff frequency with respect to the frequency, and the second cutoff frequency is smaller than the first cutoff frequency.

TECHNICAL FIELD

The present invention relates to an ultrasonic detection device and anultrasonic diagnostic device, and more particularly, to a capacitiveultrasonic detection device and an ultrasonic diagnostic device usingthe same.

BACKGROUND ART

Up to now, a capacitive ultrasonic detection device which includes acell having electrodes disposed at an interval has been known (refer toU.S. Pat. No. 6,430,109). In particular, in recent years, capacitivemicro-machined ultrasonic transducers (CMUT) using a micromachiningtechnology have been actively researched. The CMUT transmits or receivesan ultrasonic wave by the aid of a lightweight vibrating membrane, andcan easily obtain an excellent broadband characteristic even if thetransducers are placed in liquid and gas. Attention has beenincreasingly paid to ultrasonic diagnosis using the CMUT, with higherprecision than that in the related-art medical diagnosis modality as ahopeful technology. The ultrasonic receiving function of the CMUT isperformed by a capacitive electromechanical transducer and an electriccircuit disposed at a later stage. An output of the prestage capacitiveelectromechanical transducer is caused by a temporal variation of anelectrostatic capacitance, and hence the output is a current output.Accordingly, it is general to use a current-voltage conversion andamplification circuit at a later stage.

On the other hand, up to now, a piezoelectric material has been mainlyused for the practical ultrasonic transducer. The resolution of thepiezoelectric type device is proportional to the frequency, and hencethe ultrasonic transducer normally has the center sensitivity in a rangeof from 3 MHz to 10 MHz. As compared with the piezoelectric type device,the CMUT has a feature of a broad frequency band. However, thepiezoelectric type is about to be replaced with the related-art generalultrasonic diagnosis sensor, and hence the center frequency of thissensor is also generally about 3 MHz to 10 MHz. However, in order toeffectively use the broad frequency band, a broad band is also requiredfor the later stage electric circuit. The frequency characteristic ofthe ultrasonic receiving function of the CMUT is generally configured asa band pass type between the cutoff frequency of the capacitanceelectromechanical transducer and the cutoff frequency of an amplifiercircuit. Therefore, the amplifier circuit having the cutoff frequencysufficiently larger than the receive band is frequently used. Regardingthis matter, “IEEE Translations on Ultrasonics, Ferroelectrics, andFrequency Control, Vol. 55, No. 2, February 2008” discloses an amplifiercircuit having a feedback resistance and a feedback capacitance, whichis a capacitance parasitically existing in a MOS transistor circuit. Asa result, the frequency band of the CMUT disclosed in theabove-mentioned publication falls within a range of from 2 MHz to 7 MHz.

SUMMARY OF INVENTION Technical Problem

Under the above-mentioned technical circumstances, in a specimen test,the ultrasonic transducers, which display not only a configuration imagebut also a function image, have been increasingly developed in recentyears. As one of the ultrasonic transducers of this type, there is anultrasonic transducer using a photoacoustic spectrometry. A frequencyband of a photoacoustic wave used in the photoacoustic spectrometry isgenerally low as compared with a frequency band of an ultrasonic waveused in an ultrasonic echo. For example, the frequency band of thephotoacoustic wave is distributed in a range of from 200 KHz to 2 MHz,which is lower than a center frequency 3.5 MHz of the ultrasonic waveused in the ultrasonic echo. For that reason, an ultrasonic transducerthat can detect a relatively low frequency band with a high sensitivityneeds to be developed.

Solution to Problem

In view of the above-mentioned problem, an ultrasonic detection deviceaccording to the present invention includes a capacitiveelectromechanical transducer, a voltage source, and an electric circuit,and has the following features. The capacitive electromechanicaltransducer includes a cell including a first electrode and a secondelectrode disposed so as to oppose with a space. The voltage source isfor developing a potential difference between the first electrode andthe second electrode. The electric circuit is for converting a current,which is caused by a change in electrostatic capacitance between thefirst electrode and the second electrode due to vibration of the secondelectrode, into a voltage. The capacitive electromechanical transducerprovides an output current with a high-pass characteristic having afirst cutoff frequency with respect to a frequency, and the electriccircuit provides an output with a low-pass characteristic having asecond cutoff frequency with respect to the frequency. Further, thesecond cutoff frequency is smaller than the first cutoff frequency.

Further, in view of the above-mentioned problem, an ultrasonicdiagnostic device according to the present invention includes theultrasonic detection device described above, a light source, and asignal processing system that processes a signal detected by theultrasonic detection device. Further, light emitted from the lightsource is applied to an object to be tested, an elastic wave generatedby a photoacoustic effect due to the light applied to the object to betested is detected by the ultrasonic detection device, and a detectionresult is processed by the signal processing system to acquireinformation on the object to be tested.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, the cutoff frequency of thecurrent-voltage conversion and amplification circuit is set to be largerthan the cutoff frequency of the capacitive electromechanicaltransducer, and those frequency characteristics are conformed to eachother for the purpose of providing the frequency characteristic of theultrasonic detection device. Therefore, the ultrasonic detection devicehaving a lower frequency band than that of the related-art ultrasonicprobe as a band can be realized. Further, the ultrasonic diagnosticdevice suitable for the photoacoustic spectrometry can be provided bythe ultrasonic detection device, the light source, and the signalprocessing system.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A, 1B, and 1C are graphs showing the respective frequencycharacteristics of a capacitive electromechanical transducer provideddisposed at a prestage of an ultrasonic detection device according tothe present invention, an electric circuit disposed at a later stagethereof, and an entire device thereof.

FIG. 2 is a diagram illustrating a configuration of an ultrasonicdetection device according to an embodiment of the present invention.

FIGS. 3A and 3B are configuration diagrams of an ultrasonic detectiondevice according to another embodiment of the present invention.

FIGS. 4A and 4B are configuration diagrams of an ultrasonic detectiondevice according to still another embodiment of the present invention.

FIG. 5 is a configuration diagram of an ultrasonic diagnostic deviceaccording to an embodiment of the present invention.

FIGS. 6A, 6B, and 6C are graphs showing frequency characteristics of arelated art.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an ultrasonic detection device and an ultrasonic diagnosticdevice according to embodiments of the present invention are described.

The significant feature of the devices according to the presentinvention resides in that a second cutoff frequency of a frequencycharacteristic of a low-pass characteristic of an output from anelectric circuit is made smaller than a first cutoff frequency of afrequency characteristic of a high-pass characteristic of an outputcurrent from a capacitive electromechanical transducer. The definitionsof those cutoff frequencies, the high-pass characteristic, and thelow-pass characteristic are described. As described later, the frequencycharacteristic of the output from the capacitive electromechanicaltransducer is maximum at the resonance frequency of a vibrating membranein a vacuum. That is, when the vibrating membrane vibrates at theresonance frequency, the maximum value of the output is obtained. Thefrequency characteristic of the present invention is the frequencycharacteristic of the band-pass characteristic between antiresonantfrequencies through the resonance frequency from a frequency (firstcutoff frequency) lowered by about 3 dB at a frequency side lower thanthe maximum frequency. In the device actually fabricated, an averagevalue in a range close to the frequency of the maximum output can beused to define the first cutoff frequency. The frequency characteristicof a capacitive mechano-electrical transducer in this case is ahigh-pass characteristic having the first cutoff frequency. That is, thehigh-pass characteristic in the present invention has a distributionthat has the gain increased at a substantially given slope with respectto the frequency in a frequency domain lower than the cutoff frequency,and is flat in a frequency domain higher than the cutoff frequency. Onthe other hand, the later stage current-voltage conversion andamplification circuit has the frequency characteristic of the low-passcharacteristic having the second cutoff frequency determined accordingto the values of the feedback resistance and the feedback capacitance.The low-pass characteristic in the present invention has a distributionthat has the gain decreased at a given slope with respect to thefrequency in a frequency domain higher than the cutoff frequency, and isflat in a frequency domain lower than the cutoff frequency. In thiscase, the second cutoff frequency means a frequency indicative of thegain lowered from the gain in the low band by about 3 dB. Specifically,the low-pass characteristic becomes “flat” in a domain lower than thecutoff frequency, and has “the gain decreased at the given slope” in thedomain higher than the cutoff frequency. Likewise, the high-passcharacteristic has “the gain increased at the given slope” in the domainlower than the cutoff frequency, and becomes “flat” in the domain higherthan the cutoff frequency. In the present invention, the “distributionthat is flat” means the given gain, but includes not only a case wherethe distribution is completely flat, but also a case where the slope ofthe gain with respect to the frequency characteristic is as small as theslope which can be ignored in the device design.

Based on the above concept, the ultrasonic detection device and theultrasonic diagnostic device according to the present invention have thebasic configuration described above. Embodiments described below can beimplemented based on the basic configuration. For example, thecapacitive electromechanical transducer includes a first electrodearranged on a substrate, a second electrode facing the first electrode,an insulator and a space which are held between the first electrode andthe second electrode, and a vibrating membrane that verticallyvibratably supports the second electrode (refer to Embodiment 1described later). Further, in the ultrasonic detection device, acapacitor including the first electrode and the second electrodeincludes multiple spaces and multiple second electrodes or vibratingmembranes. The characteristic of the output current of the capacitiveelectromechanical transducer is determined according to factorsincluding the average of the mechanical characteristics of the multiplesecond electrodes or vibrating membranes and the electrostaticcapacitance of the capacitor (refer to Embodiment 2 described later). Inthe ultrasonic detection device, the capacitors are arrangedtwo-dimensionally, and vibration information of the second electrodes orvibrating membranes can be detected two-dimensionally (refer toEmbodiment 3 described later).

A principle of the present invention is described. In the capacitiveelectromechanical transducer, it is not easy to set the center frequencyband to about 1 MHz from the viewpoint of design. This is because, inorder to set the center frequency band to about 1 MHz, the hardness ofthe second electrode or the vibrating membrane, which is a membrane,needs to be softened (reduced in spring constant), and there is anadverse effect that the sensitivity is deteriorated with the softenedmembrane, with the result that the degree of freedom of design islimited. That is, the membrane deflection becomes large, thereby makingit difficult to produce a narrow space structure for high sensitivity.Further, an applied voltage to the electrodes is required to bedecreased, and therefore the sensitivity is lowered. Under thecircumstances, the present invention provides a system in which, under astate in which the capacitive electromechanical transducer is designedto have the center frequency at the higher frequency side from about 1MHz, the cutoff frequency of the later stage electric circuit isadjusted so that the center frequency band is about 1 MHz in total. Insuch an intended relatively low frequency band, the adjustment of thecutoff frequency of the electric circuit while keeping a highamplification gain to some degree is relatively easy with hardlyadversely affecting the other characteristics. On the contrary, anincrease in the cutoff frequency while keeping the high amplificationgain of the electric circuit is equivalent to a reduction in thefeedback resistance or a reduction in the feedback capacitance. Thiscauses the deterioration of the S/N ratio, or the deterioration of theentire sensitivity. Alternatively, there is a limit of the circuitperformance.

The above configuration is further described with reference to FIGS. 1A,1B, and 1C. When a change in the electrostatic capacitance is subjectedto parallel plane approximation, a frequency characteristic 1 of anoutput current I of the capacitive electromechanical transducer to aninput sound pressure (pressure of an input elastic wave) (refer to FIG.1A) is formulated by the following Expression (1).

I=P/[(Zm+Zr)/(εA*V _(b) /d ² +jωC]  (1)

where e is a dielectric constant of vacuum, A is an area of an electrode(refer to an upper electrode 7 described later) of the electromechanicaltransducer, V_(b) is a bias voltage applied between electrodes, d is avacuum equivalent distance between the electrodes, P is the input soundpressure, Zm is a mechanical impedance of the vibrating membrane (referto a vibrating membrane 8 described later), Zr is an acoustic impedanceof a medium around the electromechanical transducer, and ω is an angularfrequency of the input sound pressure, and C is an entire electrostaticcapacitance. In this expression, because the entire electrostaticcapacitance is relatively small, it can be said that the frequencyfunction is the mechanical impedance Zm of the vibrating membrane.

Zm is represented by the following Expression (2).

Zm=j*km*{(ω/ω₀ ²)−1/ω}  (2)

where km is the spring constant of the vibrating membrane, and thevibrating membrane is displaced in proportion to a pressure P in adomain of the frequency lower than the resonance angular frequency ω₀(which is close to the first cutoff frequency 2; refer to FIG. 1A). Zmapproaches 0 in inverse proportion to the frequency in a range of fromthe low frequency domain to the resonance frequency. From this fact, inthe frequency domain smaller than the resonance frequency of thevibrating membrane, the output current frequency characteristic 1becomes a primary characteristic of the frequency. The curves of thefrequency characteristics in FIGS. 1A to 1C are simplified and visuallyfacilitated for description of the principle. In fact, for example, theshape is a little more deformed and gently changed in the vicinity of ashoulder portion, and the cutoff frequency is not always placed at acorner of the shoulder portion as shown in the graphs. The axis ofabscissa of FIG. 1A represents a logarithmically expressed frequency,and the above-mentioned primary characteristic means a primarycharacteristic with respect to the logarithmically expressed frequency.Likewise, the above-mentioned inverse proportion means the inverseproportion to the logarithmically expressed frequency.

Further, as understood from the above Expression (1), the output currentfrequency characteristic 1 depends on not only the mechanical impedanceZm of the vibrating membrane, but also the acoustic impedance Zrconstant in the use environment. Normally, the capacitiveelectromechanical transducer is frequently immersed in the liquid inuse. The acoustic impedance of liquid is larger than the mechanicalimpedance of the vibrating membrane. In this case, the acousticimpedance of liquid is dominative in the frequency characteristic 1. Asdescribed above, the frequency at which the mechanical impedance Zm ofthe vibrating membrane is 0 is the resonance frequency of the vibratingmembrane. In this case, the output current frequency characteristic 1 isa maximum value. The mechanical impedance of the vibrating membrane isoriginally the antiresonant frequency of the vibrating membrane, andbecomes infinite. However, when the capacitive electromechanicaltransducer is used in the vicinity of a domain lower than the resonancefrequency, because the antiresonant frequency is irrelevant, the domainin the vicinity of the antiresonant frequency is omitted in the outputcurrent frequency characteristic 1 of FIG. 1A. Considering theabove-mentioned matters comprehensively, the output current frequencycharacteristic 1 represented by the above-mentioned Expression (1) isshown in FIG. 1A.

On the other hand, a frequency characteristic 3 of the current-voltageconversion and amplification circuit (refer to FIG. 1B) is formulated inthe following Expression (3), and a second cutoff frequency 4 isrepresented by the following Expression (4).

G=Rf/(1+jωRf*Cf)  (3)

f=1/(2nRf*Cf)  (4)

where G is a gain of the electric circuit, Rf is the feedbackresistance, Cf is the feedback capacitance, and f and ω are thefrequency and the angular frequency of the input current. The electriccircuit used in the configuration of the present invention is desired tobe configured by an electric circuit having the primary characteristicwith respect to the frequency as represented by Expression (3)(characteristic related to the logarithmically expressed frequency aswith the above-mentioned frequency characteristic 1), and is notpreferred to be configured by a circuit having a high-ordercharacteristic.

In the present invention, the frequency characteristic 1 of the outputcurrent of the capacitive electromechanical transducer and the frequencycharacteristic 3 of the output of the electric circuit are combinedtogether to realize the ultrasonic detection device having a lowerfrequency band than that of the conventional ultrasonic probe as a band.In the combination, in order to realize the ultrasonic detection devicehaving an intended characteristic 5 (refer to FIG. 1C), a second cutofffrequency 4 of the frequency characteristic 3 of the output of theelectric circuit is made smaller than the first cutoff frequency 2 ofthe frequency characteristic 1 of the output current of the capacitiveelectromechanical transducer. The reason is described above.

In this way, the output current frequency characteristic 1 of thecapacitive electromechanical transducer is combined with the frequencycharacteristic 3 of the current-voltage conversion and amplificationcircuit to provide an output frequency characteristic 5 of theultrasonic detection device. As shown in FIG. 1C, the effectivefrequency band is between a low band side cutoff frequency 101 and ahigh band side cutoff frequency 102. In this case, the low band sidecutoff frequency 101 and the high band side cutoff frequency 102 are notalways conformed to the second cutoff frequency 4 and the first cutofffrequency 3, respectively. This is because when the first cutofffrequency 3 and the second cutoff frequency 4 are close to each other,it is difficult that the output frequency characteristic 5 of theultrasonic detection device becomes substantially flat in distributionbetween the low band side cutoff frequency 101 and the high band sidecutoff frequency 102. The frequency characteristic 1 and the frequencycharacteristic 3 may be designed to provide a substantially flatdistribution while keeping a given magnitude between the low band sidecutoff frequency 101 and the high band side cutoff frequency 102. Inorder to achieve this, for example, it is preferred that the slope of aninclined portion of the frequency characteristic 1 and the slope of aninclined portion of the frequency characteristic 3 be opposite in signto each other and equal in absolute value to each other as much aspossible. Further, it is preferred that the gain of the frequencycharacteristic 3 be increased.

From the above viewpoint, it is preferred that, in the ultrasonicdetection device having the broad band and the high sensitivity, forexample, a frequency that is the geometric mean of the cutoff frequency2 and the cutoff frequency 4 be in a range of from 0.4 MHz to 1.0 MHz,and have the frequency characteristic 5 shown in FIG. 1C. In the casewhere a value obtained by dividing the flat frequency band of thefrequency characteristic 5 by its center value is 130%, when thegeometric mean of the cutoff frequency 2 and the cutoff frequency 4 isset to 0.4 MHz, an ultrasonic wave of 0.2 MHz can be detected. Likewise,when the geometric mean of the cutoff frequency 2 and the cutofffrequency 4 is set to 1.0 MHz, an ultrasonic wave of 2.0 MHz can bedetected.

Up to now, when normal semiconductor or micromachining related materialis used, in a liquid through which an ultrasonic wave from a living boyeasily penetrates, the frequency characteristic of the capacitiveelectromechanical transducer is saturated and stable at about 3 MHz orhigher. However, as described above, it is difficult to obtain the CMUThaving the center in the vicinity of 1 MHz, and the high sensitivity.According to the present invention using the above-mentioned principle,such difficulty can be also eliminated. For comparison, the frequencycharacteristic of the conventional capacitive electromechanicaltransducer, the frequency characteristic of the conventional electriccircuit, and the frequency characteristic of the conventional ultrasonicdetection device are shown in FIGS. 6A, 6B, and 6C. The frequencycharacteristic of FIG. 6A is not substantially different from thefrequency characteristic of FIG. 1A. However, in the frequencycharacteristic of FIG. 6B, the cutoff frequency 4 is at the highfrequency side, and the gain is low totally as compared with thefrequency characteristic of FIG. 1B. As a result, in the frequencycharacteristic of FIG. 6C, the low band side cutoff frequency 101 andthe high band side cutoff frequency 102 range at the high frequencyside, for example, 3 MHz to 10 MHz.

Hereinafter, embodiments having the configurations of the capacitiveelectromechanical transducer and the current-voltage conversion andamplification circuit embodied based on the above-mentioned principleare described with reference to the drawings.

Embodiment 1

An ultrasonic detection device according to Embodiment 1 is described.The configurations of a capacitive electromechanical transducer 6(hereinafter, also called “cell”) and an electric circuit 14 accordingto this embodiment are illustrated in FIG. 2. The capacitiveelectromechanical transducer 6 indicated as one cell includes an upperelectrode 7, a vibrating membrane 8, a cavity 9, an insulating layer 10,support parts 11 that support the vibrating membrane 8, a lowerelectrode 12, and a substrate 13 that supports those members. Theelectric circuit 14 includes a resistor R1 connected to the upperelectrode 7 and the lower electrode 12, and an operational amplifierhaving a feedback resistor Rf and a feedback capacitor Cf. Thetransducer 6 and the electric circuit 14 are configured to have theabove-mentioned frequency characteristic.

FIG. 2 is an example of the configurations. When the vibrating membrane8 is made of an insulator, the insulating layer 10 may or may not beprovided. In this case, the vibrating membrane 8 and the support parts11 may be made of the same material. The insulating layer 10 and thesupport parts 11 may be made of the same material. Structurally, theupper electrode 7 and the vibrating membrane 8 are bonded to each other,and vibrate integrally. From the viewpoint of improving the sensitivity,it is desired that the cavity 9 be maintained at a pressure lower thanthe atmospheric pressure. When the substrate 13 is formed of aconductive substrate such as a semiconductor substrate made of silicon,the substrate 13 and the lower electrode 12 may be integrated together.The output current frequency characteristic 1 depends on the mechanicalimpedance of the vibrating membrane 8 and the acoustic impedance of theuse environment. Normally, the capacitive electromechanical transduceris frequently immersed in liquid 18 in use. The acoustic impedance ofthe liquid 18 is larger than the mechanical impedance of the vibratingmembrane 8. Specifically, the liquid is water, an ultrasonic diagnosisgrease, or an oil such as ricinus oil.

In general, the upper electrode 7 and the lower electrode 12 are desiredto be made of metal, but may be made of low-resistant semiconductor. Forexample, the upper electrode 7 which is a second electrode can be madeof at least one material of electric conductor selected from Al, Cr, Ti,Au, Pt, Cu, Ag, W, Mo, Ta, and Ni, semiconductor such as Si, and alloyselected from AlSi, AlCu, AlTi, MoW, AlCr, TiN, and AlSiCu. Further, theupper electrode 7 is disposed on at least one portion of an uppersurface, a rear surface, and an interior of the vibrating membrane 8.Alternatively, the vibrating membrane 8 can be structured to serve alsoas the upper electrode 7 when the vibrating membrane 8 is made ofelectric conductor or semiconductor. The lower electrode 12 which is afirst electrode can be made of the same electric conductor orsemiconductor as that of the upper electrode 7. The electrode materialsof the lower electrode 12 and the upper electrode 7 may be differentfrom each other.

The dimensions of the respective parts in this embodiment areexemplified as follows. For example, a height of the cavity 9 is about100 nm, but may be in a range of from 10 nm to 500 nm. A length of onepiece of the cavity 9 is, for example, in a range of from 10 μm to 200μm. The vibrating membrane 8 is made of, for example, SiN, but may bemade of other insulating materials. The cavity 9 is held in a pressurereduction state with respect to the atmospheric pressure, and thevibrating membrane 8 is slightly recessed. The vibrating membrane andthe electrodes are, for example, square, but may be circular orpolygonal. The shape of the cavity 9 of the cell is also, for example,square, but may be other shapes.

During receiving operation, a DC voltage V is applied by a voltagesource 15 in order to develop a potential difference between the upperelectrode 7 and the lower electrode 12 of the cell 6 of the ultrasonicdetection device. In receiving an ultrasonic wave, the vibratingmembrane 8 oscillates, and a current is caused to flow by the amount aslarge as a change in the capacitance caused by the vibration. Thecurrent is amplified by the current-voltage conversion and amplificationcircuit 14.

Embodiment 2

An ultrasonic detection device according to Embodiment 2 is described.The configuration of this embodiment is illustrated in FIGS. 3A and 3B.FIG. 3A illustrates a conceptual cross-sectional view of the ultrasonicdetection device, and FIG. 3B illustrates a plan view of an element 20.Portions indicated by broken lines in FIGS. 3A and 3B represent thatdrawing of the structure is omitted except for a perspective portion ofthe cell 6. In this embodiment, the multiple cells 6 are arranged on thesubstrate 13. The structures of each cell 6 and the electric circuit 14are the same as those described in Embodiment 1. The upper electrodes 7and the lower electrodes 12 of the multiple cells 6 are electricallyconnected by electrode coupling wiring parts 16 and 17, respectively, sothat the multiple cells 6 are rendered conductive to each other. Asillustrated in FIG. 3B, the cells 6 are arranged two-dimensionally atregular intervals to form one element 20. The device is used, forexample, under a state in which the upper electrodes 7 of the element 20are brought into contact with the liquid 18 excellent in propagation ofthe ultrasonic wave. From the viewpoint of the detection sensitivity andthe facility of signal processing, it is desired that the mechanicalcharacteristics of the vibrating membranes 8 and the depths of thecavities 9 be uniform in the multiple cells 6. Within the element 20,the arrangement of the cells 6 is in a square lattice in the illustratedexample, but may be in a zigzag shape or a hexagonal close-packed shape.The arrangement form and the number of cells 6 within the element 20 canbe appropriately determined as occasion demands. In the illustratedexample, the shape of the vibrating membranes 8 is circular, but may bepolygonal. In this way, in this embodiment, the capacitors each made upof the lower electrode 12 (first electrode) and the upper electrode 7(second electrode) include the multiple spaces 9 and the multiple secondelectrodes or vibrating membranes 8. The frequency characteristic of theoutput current of the element 20 is determined according to factorsincluding the average of the mechanical characteristics of the multiplesecond electrodes or vibrating membranes 8, and the electrostaticcapacitance of the capacitor.

In this embodiment, the domain where the multiple upper electrodes 7 arerendered conductive to each other forms an ultrasonic detection domain,and increases the sensitivity as compared with that of Embodiment 1including one cell. In this embodiment, it may be said that the element20 including the multiple cells configures one capacitiveelectromechanical transducer. In this case, the frequency characteristic1 of the capacitive electromechanical transducer (refer to FIG. 1A) isdetermined, as described above, according to the average value of themechanical characteristics of the multiple vibrating membranes 8.Further, the magnitude of the current output of the element 20 issubstantially proportional to the total area of the upper electrodes 7on the multiple vibrating membranes 8. Other configurations areidentical with those in Embodiment 1.

Embodiment 3

An ultrasonic detection device according to Embodiment 3 is described.The configuration of this embodiment is illustrated in FIGS. 4A and 4B.FIG. 4A which is a top view is a conceptual cross-sectional viewillustrating the entire configuration of an ultrasonic detection device32, and also illustrates electric coupling including wirings 31 andcurrent-voltage conversion and amplification circuits 14. FIG. 4B is aplan view of the ultrasonic detection device 32. In FIG. 4B, the wirings31 and the current-voltage conversion and amplification circuits 14 arehidden below. However, those members may be arranged laterally.Similarly, in FIGS. 4A and 4B, portions indicated by broken lines showthat drawing of the structure is omitted. An ultrasonic detection device30 according to this embodiment is configured so that the elements 20 ofEmbodiment 2 are arranged two-dimensionally. Any one of the upperelectrodes 7 connected by the wiring parts 16 and lower electrodes 12connected by the wiring parts 17 are electrically separated in each ofthe elements 20. Similarly, in this embodiment, the upper electrodes 16are brought into contact with the liquid 18 excellent in propagation ofthe ultrasonic wave. An output of each element 20 is transmitted to eachcurrent-voltage conversion and amplification circuit 14 by each wiring31 to conduct voltage conversion. As a result, the ultrasonic signal canbe detected as a two-dimensional distribution. Similarly, in thisexample, the frequency characteristic 1 of each element 20 is determinedaccording to the average value of the mechanical characteristics of themultiple vibrating membranes 8. Further, the amplitude of the currentoutput of each element 20 is substantially proportional to the totalarea of the multiple upper electrodes 7. In the ultrasonic detectiondevice according to this embodiment, and the capacitors are arrangedtwo-dimensionally, and the vibrating information of the secondelectrodes or the vibrating membranes 8 can be detectedtwo-dimensionally. Other configurations are identical with those inEmbodiment 1.

Incidentally, the configuration of the above-mentioned embodiment can beused as a device that generates a sound wave. A voltage obtained bysuperimposing a minute AC voltage on the DC voltage is applied betweenthe upper electrodes 7 (or the upper electrode coupling wiring parts 16)and the lower electrodes 12 (or the lower electrode coupling wiringparts 17) by the voltage source 15 to forcedly vibrate the vibratingmembranes 8 for generation of the sound wave. In this case, thefrequency characteristic mainly has the same transmission characteristicas that of the output current frequency characteristic 1 of thecapacitive electromechanical transducer. The sound wave generationdevice arranges the vibrating membranes 8 two-dimensionally as in theabove-mentioned Embodiment 2 or Embodiment 3 to generate a larger soundwave. Further, when the generation area is increased, the directionalityof the sound wave can be increased, and the diffraction can bedecreased.

Embodiment 4

An ultrasonic diagnostic device according to Embodiment 4 is described.The configuration of this embodiment is illustrated in FIG. 5. Light 41emitted from a light source 40 is propagated and applied to a bodytissue 42, to thereby generate an ultrasonic wave 43 which is called“photoacoustic wave”. That is, the light is absorbed by a portion largein light absorption coefficient that exists within the body tissue, andthat portion is heated. Then, the heated portion is expanded, and anelastic wave is generated by expansion. The ultrasonic wave 43 is, forexample, about 200 kHz to 2 MHz as described above although beingdifferent according to material or individual configuring the bodytissue. The ultrasonic wave (photoacoustic wave) 43 passes through theliquid 18 excellent in propagation of the ultrasonic wave, and isdetected by the ultrasonic detection device 32. The current-voltageconverted and amplified signal is transmitted to a signal processingsystem 45 through a signal bus 44. A signal of the detection result isprocessed by the signal processing system 45 to extract the bodyinformation. When the ultrasonic detection device 32 is configured as inthe above-mentioned Embodiment 3, the two-dimensional ultrasonicdistribution can be detected, and a wide-range distribution can becaptured by scanning the ultrasonic detection device 32. Because theultrasonic wave has a sound speed, a time difference of arrival waves(time waveform) may be analyzed to obtain time information, andinformation in the depth direction can be acquired. In this case, areconstruction function can be provided to the signal processing system45 so as to extract the three-dimensional body information. Further, thereceived signal may be subjected to Fourier transform to obtain thefrequency characteristic so as to acquire an image or the like.

As described above, a technique by which a cross-sectional image or athree-dimensional image of a specimen (object to be tested) is acquiredby using the photoacoustic effect has been generally known as aphotoacoustic tomography, and called “PAT technology” from its initial.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-254752, filed Nov. 6, 2009, which is hereby incorporated byreference herein in its entirety.

1. An ultrasonic detection device, comprising: a capacitiveelectromechanical transducer including a cell that includes a firstelectrode and a second electrode disposed so as to oppose with a space;a voltage source for developing a potential difference between the firstelectrode and the second electrode; and an electric circuit forconverting a current, which is caused by a change in electrostaticcapacitance between the first electrode and the second electrode due tovibration of the second electrode, into a voltage, wherein thecapacitive electromechanical transducer provides an output current witha high-pass characteristic having a first cutoff frequency with respectto a frequency, wherein the electric circuit provides an output with alow-pass characteristic having a second cutoff frequency with respect tothe frequency, and wherein the second cutoff frequency is smaller thanthe first cutoff frequency.
 2. The ultrasonic detection device accordingto claim 1, wherein the capacitive electromechanical transducer includesa first electrode arranged on a substrate, a second electrode which isdisposed so as to be opposed to the first electrode, an insulator and aspace held between the first electrode and the second electrode, and avibrating membrane that vertically vibratably supports the secondelectrode.
 3. The ultrasonic detection device according to claim 1,wherein, in a frequency characteristic of the ultrasonic detectiondevice, a geometric mean of the first cutoff frequency and the secondcutoff frequency is in a range of from 0.4 MHz to 1.0 MHz.
 4. Theultrasonic detection device according to claim 1, wherein the firstelectrode and the second electrode constitute respective capacitorsincluding multiple spaces and multiple second electrodes or multiplevibrating membranes, and wherein a characteristic of the output currentof the capacitive electromechanical transducer is determined accordingto factors including an average of mechanical characteristics of themultiple second electrodes or the multiple vibrating membranes, andelectrostatic capacitances of the capacitors.
 5. The ultrasonicdetection device according to claim 4, wherein the capacitors arearranged two-dimensionally, and wherein vibrating information of thesecond electrode or the vibrating membrane can be detectedtwo-dimensionally.
 6. An ultrasonic diagnostic device, comprising: theultrasonic detection device according to claim 1; a light source; and asignal processing system that processes a signal detected by theultrasonic detection device, wherein light emitted from the light sourceis applied to an object to be tested, an elastic wave generated by aphotoacoustic effect due to the light applied to the object to be testedis detected by the ultrasonic detection device, and a detection resultis processed by the signal processing system to acquire information onthe object to be tested.